Method For Producing A Single Crystal Of Semiconductor Material

ABSTRACT

A single crystal of semiconductor material is produced by a method of melting semiconductor material granules by means of a first induction heating coil on a dish with a run-off tube consisting of the semiconductor material, forming a melt of molten granules which extends from the run-off tube in the form of a melt neck and a melt waist to a phase boundary, delivering heat to the melt by means of a second induction heating coil which has an opening through which the melt neck passes, crystallizing the melt at the phase boundary, and delivering a cooling gas to the run-off tube and to the melt neck in order to control the axial position of an interface between the run-off tube and the melt neck.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German application DE 10 2008038810.6 filed Aug. 13, 2008, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing a single crystal ofsemiconductor material, in which semiconductor material granules aremelted on a dish that has a run-off tube consisting of the semiconductormaterial, and a melt of molten granules is formed, which extends fromthe run-off tube in the form of a melt neck and a melt waist to a phaseboundary, and heat is delivered to the melt by means of an inductionheating coil which has an opening through which the melt neck passes,and the melt is crystallized at the phase boundary.

2. Background Art

Such a method is described, for example, in US 2003145781 A. It makes itpossible to produce a single crystal of semiconductor material withgranules as raw material. FIG. 4 of US 2003145781 A shows a device whichis suitable for carrying out the method. The granules are melted on adish, in the middle of which there is a passage opening extended to arun-off tube. A first induction heating coil, arranged above the dish,is used to melt the granules. The molten granules form initially a film,and in the further course of the method, a melt which crystallizes at aphase boundary and thereby increases the volume of the growing singlecrystal. The crystallizing volume is compensated for by a correspondingvolume of newly melted granules. The melt extends from the run-off tubeto the phase boundary, at which the single crystal grows. In the regionof the run-off tube, it has the form of a melt neck which passes throughthe opening of a second induction heating coil and merges into a widermelt waist, which lies on the growing single crystal. With the aid ofthe second induction heating coil, heat is delivered to the melt inorder to control the growth of the single crystal.

Since the run-off tube consists of the semiconductor material, it may becaused to melt by the second induction heating coil if the energy inputis correspondingly high. On the other hand, the run-off tube may growdownward if the energy provided by the second induction heating coil isnot sufficient to keep the melt liquid in the region of the run-offtube. The position of the interface between the run-off tube and themelt must not however be displaced arbitrarily far axially, i.e. upwardor downward. If the interface migrates too far upward because therun-off tube is melted, the volume of the melt neck increases and therisk arises that the melt will touch the second induction heating coilor the melt neck will become too thin and break. If the interfacemigrates too far downward because the run-off tube grows in thisdirection, the risk arises that the run-off tube will freeze up andblock the melt flow. These two situations must not occur, because theyprevent further growth of the single crystal.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method so thatmore effective control of the axial position of the interface ispossible. This and other objects are achieved by a method for producinga single crystal of semiconductor material, comprising melting ofsemiconductor material granules by means of a first induction heatingcoil on a dish with a run-off tube consisting of the semiconductormaterial, forming a melt of molten granules which extends from therun-off tube in the form of a melt neck and a melt waist to a phaseboundary; delivering heat to the melt by means of a second inductionheating coil which has an opening through which the melt neck passes;crystallizing the melt at the phase boundary; and delivering a coolinggas to the run-off tube and to the melt neck in order to control theaxial position of an interface between the run-off tube and the meltneck. The objects are furthermore achieved by a device for producing asingle crystal of semiconductor material, which comprises a dish forreceiving granules of semiconductor material, the dish having an openingat its center, which is extended to a run-off tube; a first inductionheating coil for melting the granules on the dish; a second inductionheating coil for transferring energy to a melt formed by the moltengranules, the second induction heating coil having a passage opening forthe melt at its center; and an instrument for the controlled delivery ofa gas into a region where a melt neck of the melt and the run-off tubetouch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a device which is particularly suitable for carrying outthe method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Granules 13 from a funnel are melted on a rotatable dish 9, in themiddle of which there is a passage opening extended to a run-off tube11. A first induction heating coil, arranged above the dish, is used tomelt the granules 13. The first induction heating coil is preferablydesigned so that radiofrequency current, which is supplied through coilterminals 5, essentially flows through a coil former 1 and segments 2.The segments are electrically conductively connected to one another attheir lower end through a thin bar 3. The coil former 1 has radiallydirected current guide slots, which force current to flow on ameandering path through the coil former. This ensures that all regionsof the surface of the rotating dish are covered uniformly by theelectromagnetic field. In an outer region, the coil former 1 has atleast one passage opening 6 for delivering the semiconductor materialgranules 13 onto the rotating dish 9. The first induction heating coilis furthermore equipped with a cooling system which, in the coil former1, comprises cooling channels 7 through which a coolant flows, forexample water. So that intense cooling of the segments 2 can also beobtained, the cooling channels continue up to the segments and areconnected together by a bridging tube 8. The bridging tube extends atthe centre of the upper side of the coil former 1 as far as the segments2, and is for example soldered or welded onto them. The bridging tube 8is wound one or more times, so that it has a sufficiently highinductance. The radiofrequency current therefore flows essentiallythrough the bar 3, which connects the segments 2, and not through thebridging tube 8. Owing to the flow of current, the field line density isparticularly high in the region of the bar and the inductive heating ofthe melt's part which lies directly opposite the bar 3 during productionof the single crystal 10, is particularly effective. Preferably the sameelectrical potential, most preferably ground potential, is applied tothe melt and the bar.

The dish 9 consists of the same semiconductor material as the granules13 and is preferably configured in a manner like the container which isdescribed in DE 102 04 178 A1, the content of which is herebyincorporated by reference. It may however also be configured as a simpleflat plate with a central run-off tube.

In the course of the method, the granules form a melt which may besubdivided into a continuous film 12, a melt neck 18 and a melt waist16. The melt crystallizes at a phase boundary 4 and thereby increasesthe volume of the growing crystal 10. The crystallizing volume iscompensated for by a corresponding volume of newly melted granules. Themelt neck 18 extends from the lower end of the run-off tube 11 as far asthe melt waist 16, and passes through the opening of a second inductionheating coil 15. The melt waist 16, which is wider than the melt neck,bears on the growing single crystal 10. With the aid of the secondinduction heating coil 15, heat is delivered to the melt in order tocontrol the growth of the single crystal 10. A shield 19, whichpreferably consists of an actively cooled metal plate, is arrangedbetween the induction heating coils in order to shield themelectromagnetically from one another. Furthermore, the shield 19 coolsthe bottom of the dish 9.

In order to carry out the method according to the invention, aninstrument is provided which permits controlled delivery of a coolinggas to the run-off tube 11 and to the melt neck 18 in the region of aninterface 17 between the run-off tube and the melt neck. In theembodiment represented, the instrument comprises a nozzle 20 throughwhich the cooling gas, preferably argon, is fed from the side to therun-off tube 11 and to the melt neck 18. The nozzle 20 is preferablyintegrated into the second induction heating coil. It may however alsobe accommodated in or on the shield 19. The instrument furthermorecomprises a camera 21 for optical acquisition of the axial position ofthe interface 17, and a controller 22 for supplying the nozzle with thecooling gas. The camera, the nozzle and the control are connected toform a control loop. The axial position of the interface is identifiedby the camera from the pronounced difference in brightness between therun-off tube and the melt. The controller, preferably a PID controller(combination of proportional, integral and differential controllers)controls the volume flow of the gas through the nozzle as a function ofthe detected position of the interface 17. If the interface 17 migratesupward beyond a tolerated upper limit position, the controller increasesthe volume flow so that semiconductor material solidifies at the end ofthe run-off tube owing to the enhanced cooling effect, and the run-offtube is lengthened. The effect of this is to displace the interface 17downward. If the interface 17 migrates downward below a tolerated lowerlimit position, the controller decreases the volume flow so that therun-off tube is melted at its lower end owing to the reduced coolingeffect. The effect of this is to displace the interface upward.

The distance from the middle of the second induction heating coil 15 tothe upper and lower limit positions is preferably not more than 10 mm,more preferably not more than 5 mm. The axial position of the interface17 is thus controlled so that the interface 17 preferably remains withina region with an axial length of less than 20 mm, more preferably 10 mm.

The control may be assisted by displacing the second induction heatingcoil 15 to the side, so that the melt neck 18 no longer passes throughthe opening of the second induction heating coil axisymmetrically withthe rotation axis of the dish and the single crystal. This measure isadvantageous particularly in the phase in which the diameter of thesingle crystal is widened to a final diameter. The closer the secondinduction heating coil comes to the melt neck, the greater is theintegral energy input i.e. the amount of energy supplied in total to themelt neck. Additional energy input takes place when the second inductionheating coil approaches the melt neck, although with lateraldisplacement of the induction heating coil the distance on one side ofthe melt neck decreases, but at the same time increases on the oppositeside of the melt neck. Lateral displacement of the second inductionheating coil, from a position in which the melt neck passesaxisymmetrically to the opening of the coil, toward the melt neck thusqualitatively has the same effect as reducing the volume flow of thecooling gas through the nozzle.

EXAMPLE

In order to demonstrate the success of the invention, a plurality ofsilicon single crystals with diameters of 70 mm, 105 mm and 150 mm wereproduced in a device according to FIG. 1.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method for producing a single crystal of semiconductor material,comprising melting semiconductor material granules by means of a firstinduction heating coil on a dish with a run-off tube consisting of thesemiconductor material; forming a melt of molten granules which extendsfrom the run-off tube in the form of a melt neck and a melt waist to aphase boundary; delivering heat to the melt by means of a secondinduction heating coil, which has an opening through which the melt neckpasses; crystallizing the melt at the phase boundary; and controllingthe axial position of an interface between the run-off tube and the meltneck by controlling the delivery of a cooling gas to the run-off tubeand to the melt neck.
 2. The method of claim 1, wherein the axialposition of the interface is controlled so that it remains in a regionwith an axial length of less than 20 mm.
 3. The method of claim 1,wherein the axial position of the interface is modified by displacingthe second induction heating coil toward the melt neck.
 4. The method ofclaim 2, wherein the axial position of the interface is modified bydisplacing the second induction heating coil toward the melt neck.
 5. Adevice for producing a single crystal of semiconductor material,comprising a dish for receiving granules of semiconductor material, thedish having an opening at its center which is extended to a run-offtube; a first induction heating coil for melting granules on the dish; asecond induction heating coil for transferring energy to a melt formedby the molten granules, the second induction heating coil having apassage opening for the melt at its center; and a gas delivery devicefor the controlled delivery of a gas into a region where a melt neck ofthe melt and the run-off tube touch.
 6. The device of claim 5, whereinthe instrument for the controlled delivery of a gas comprises a camera,a controller and a nozzle.